Modern reactors used to convert heavy hydrocarbonaceous feedstocks such as petroleum residuum ("resid") to lighter, more valuable products often employ slurry-type or ebullated bed hydroconversion processes. In both slurry-type and ebullated bed reactors, feedstock typically is added to a reactor vessel alone or together with hydrogen through an inlet pipe or sparger. The pipe or sparger typically is located in a lower portion of the reactor vessel in a liquid mixing region.
In ebullated bed reactors, a mixture of process liquids and hydrogen from the liquid mixing region is forced upwardly through a distributor plate containing a plurality of bubble-capped risers. Feedstock, hydrogen, and recycled liquid are forced upwardly through the risers to expand a bed of supported catalyst located above the distributor plate. Maintaining the catalyst bed in a properly expanded condition requires that the liquids supporting the bed exhibit a generally symmetric liquid velocity distribution in the expanded bed region of the reactor.
In slurry-type reactors, a distributor plate and bubble-capped risers are not needed to prevent supported catalyst from falling into the liquid mixing region. Nevertheless, many slurry-type reactors resemble ebullated bed reactors in that fresh feedstock, recycled liquids and liquid or colloidal catalyst are mixed together in a liquid mixing region in the lower end of a reactor. The mixture is then forced upwardly through a distributor plate or other device intended to provide a desired liquid velocity distribution in the region of the reactor located above the liquid mixing zone.
Liquids moving upwardly in slurry-type or ebullated bed reactors have a velocity that preferably is equal or nearly equal at all points located within a cross-section of the reactor at a given height. If asymmetries in the reactor velocity profile occur, mixing will not be uniform within the reactor. In ebullated bed reactors, insufficient liquid velocity may prevent the catalyst bed from expanding to the desired height or cause catalyst to accumulate or "slump" in areas of low liquid velocity. Catalyst slumping can in turn result in the formation of reactor hot spots and coke accumulation.
Various structures have been employed to minimize deviations in the liquid velocity profile of slurry-type or ebullated bed reactors. For example, U.S. Pat. No. 4,444,653 to Euzen discloses a plurality of inlet spargers which disperse a reactor feedstock through a plurality of relatively small discharge orifices located along the spargers. While these sparger designs may be beneficial in some applications, the use of spargers with large numbers of relatively small holes such as those disclosed by Euzen can be problematic when the reactor charge is a heavy hydrocarbonaceous feedstock such as a petroleum resid, as relatively small holes can easily become plugged in a resid hydrotreating environment. Euzen's designs also are subject to the momentum-related problems discussed in depth below.
Another method for improving the flow distribution of liquids in an ebullated bed reactor is disclosed in U.S. Pat. No. 4,702,891 to Li. This method employs a radially-symmetric recycle liquid inlet nozzle located along the centerline of a generally cylindrical reactor vessel and in a liquid mixing region below the reactor's distributor plate. Feedstock and hydrogen are introduced into the same region through a ring sparger located above the recycle nozzle. As is common in many such reactors, feedstock appears to be required to enter the reactor in an asymmetric manner because of mechanical constraints inherent in the reactor design. While Li's design may be useful in some instances, our experimental work shows that the use of a ring sparger such as in the Li patent introduces sufficient unwanted horizontal momentum into fluids present below the distributor plate to cause serious deviations in the liquid velocity profile above the distributor plate. Specifically, although Li's horizontal sparger includes downwardly-directed discharge apertures, horizontal motion through the sparger causes the discharged feedstock to retain an undesired horizontal momentum component. As our experimental data will show, this undesired horizontal momentum component can seriously degrade the liquid superficial velocity profile in the reactor region located above the inlet sparger. This effect has been found to occur even though recycled liquids are introduced into the liquid region in a manner which appears to introduce little unwanted recycled liquid momentum components.
What is needed is a means for introducing feedstock into a reactor that minimizes asymmetry in the liquid velocity profile in a reaction zone of the reactor.